1. Field of the Invention
This invention relates to a signal analyzer apparatus with an analog partial sweep function and, more particularly, to a spectrum analyzer which, when a stable signal such as a carrier wave signal and a variation signal such as noise or side-band wave adjacent to the stable signal are monitored, allows an easy monitoring of a variation signal while monitoring the relationship with the stable signal, in an apparatus used for spectrum analysis of a signal.
Also the invention relates to the following spectrum analyzer. In a spectrum analyzer for displaying a frequency along the abscissa and an analyzed signal level along the ordinate on a display screen, when a given frequency range (fi to fh) is set and carrier wave fc and side-band wave fd within the frequency range are monitored, an operator often wants to store the value of carrier wave fc and to monitor a variation in side-band wave fd. In this case, the spectrum analyzer of this invention can designate and sweep-measure only a frequency range (fm to fn, where fi&lt;fm, fn&lt;fh) around the side-band wave, and can store and display a spectrum in frequency ranges (fi to fm) and (fn to fh) excluding the range (fm to fn) (this function will be referred to as a partial sweep function hereinafter).
2. Description of the Related Art
The partial sweep function cannot be performed by a conventional spectrum analyzer. In the conventional spectrum analyzer, carrier wave fc as shown in FIG. 5A is monitored. At this time, if side-band wave fd is present and a variation in wave fd is to be monitored, a frequency range (fm to fn) is set and monitored, as shown in FIG. 5B. In this case, the side-band wave is enlarged and monitored along the frequency axis.
Meanwhile, a network analyzer is known in which a signal of a known frequency is supplied to an object to be measured and the output signal from the object is analyzed to evaluate characteristics of the object. In the network analyzer (MS420B/K, MS560J, MS620J, available from ANRITSU CORPORATION), a partial sweep operation is performed. FIG. 6 shows the arrangement of the main part of the network analyzer, and its operation will be described with reference to FIG. 6.
Local oscillator 57 comprises a frequency synthesizer and oscillates in accordance with data (binary digital signal) according to a frequency range set by measurement frequency range setting unit 58. Local oscillator 57 sends its output signal to mixer 51, so that an input signal within a predetermined frequency range is converted into an intermediate frequency (IF) signal. The IF signal is detected by detector 53 through IF circuit 52. Thereafter, the detection output from detector 53 is converted from an analog signal to a digital signal by A/D converter 54. The digital signal output from A/D converter 54 is stored in memory 55 in correspondence with frequency data from measurement frequency range setting unit 58, and is then displayed on display 56. In this arrangement, a partial sweep operation is performed as follows. This operation will be described with reference to FIGS. 5A and 5B.
A desired entire frequency range (fi to fh) on the frequency axis is designated by measurement frequency range setting unit 58, and an input signal within the range is measured and displayed on a display screen (this operation will be referred to as an entire frequency sweep operation hereinafter).
Partial sweep switching unit 59 is then turned on, and measurement frequency range setting unit 58 sets a desired partial frequency range (fm to fn). Setting unit 58 digitally sweeps local oscillator 57 so that only an input signal within the partial frequency range (fm to fn) is received at the same frequency step as in the entire frequency sweep mode. Setting unit 58 controls memory 55, so that only a signal level corresponding to the partial frequency range (fm to fn) is updated and displayed on display 56 for every sweep. The values corresponding to other frequency ranges (fi to fn) and (fn to fh) in the entire frequency sweep mode are left unchanged and stored in memory 55, and are displayed on display 56.
In this manner, in the network analyzer having a memory for digital storage and display, and local oscillator 57 comprising a frequency synthesizer capable of easy frequency control based on a binary digital signal, a digital partial sweep operation can be easily performed.
In the conventional spectrum analyzer, when a variation in side-band wave fd is measured while performing a frequency sweep operation shown in FIG. 5A, a wide-band sweep operation must be performed and a sweep time becomes too long. In this case, a variation earlier than the sweep time cannot be monitored. When an enlarged sweep operation is performed, as shown in FIG. 5B, carrier wave fc does not appear on the display screen, and the relationship between carrier wave fc and the varying side-band wave cannot be monitored on the single display screen. Therefore, values such as the frequency and level of carrier wave fc must be separately recorded, resulting in inconvenience.
The partial sweep operation performed by the network analyzer achieves an object from the viewpoint of the display screen. However, this technique poses problems in frequency control, as will be described later, and cannot be used in the spectrum analyzer.
(1) Since the spectrum analyzer searches and measures a signal of an unknown frequency, it is not suitable for the digital sweep operation of the network analyzer. For example, assuming that a measurement frequency range between 1 to 2 GHz is covered by 500 points, if the digital sweep operation is performed, a frequency step between adjacent points corresponds to 2 MHz. For this reason, when a resolution bandwidth is small, a spectrum present in a 2-MHz interval may be omitted inadvertently. In order to prevent this, a sweep operation must be performed using a smaller frequency step than the resolution bandwidth, resulting in a bulky, high-cost arrangement. In addition, a measurement speed is decreased.
(2) Since the spectrum analyzer must monitor a variety of modulated signals, it must have a high measurement speed.
However, the measurement speed of the network analyzer is almost determined by a PLL (Phase Locked Loop) response time of a synthesizer used as a local oscillator. For example, if the frequency axis of the display screen is divided into 500 points to perform measurement, it takes about 2 seconds. The PLL response time must be 100 ms or less in a spectrum analyzer.
As described above, when the spectrum analyzer is arranged using the frequency synthesizer as the local oscillator, problems (1) and (2) impair the original performance and functions of the spectrum analyzer.